abc's john donvan has the story. >> reporter: hitting the fast food drive-in for science. >> can i get five soft tacos. two sausage burritos. >> reporter: over and over and over again. it has been dave's life the past three months or so as has been eating the food. a precisely measured, 1,000 calories a day, extra meal daily, fast food only. >> how many calories? >> 770. >> reporter: why? this man is paying them to. dr. samuel klein a researcher at washington medical school in st. louis trying to understand why weight gain ladies to diseases like diabetes and hypertension and how that relates to fat in the liver and muscles. at some point, research on rats alone just is not enough. somebody has to eat the food? >> it has to come to people. >> announcer: attention overweight volunteers. >> reporter: a radio ad offered a cash incentive up to $3,500 depending how long it would take. >> once i got to work i called right away. >> reporter: so did this nurse, dawn freeman. >> it took a month. >> reporter: to get approved? >> to get approved. >> reporter: so they ate every day from one of fiv